The present invention relates to a magnetron for a microwave oven and, more particularly, to a magnetron in which a magnetic field distribution in its interaction space is improved to suppress generation of a relatively low-frequency line conducted noise component (hereinafter called line noise or line noise component).
In a magnetron, it is known that a distribution of a magnetic field applied to an interaction space greatly influences oscillation of the magnetron. Ideally, the magnetic field distribution in the interaction space should be such that magnetic flux is perfectly parallel to the tube axis and has a uniform density over the entire region of the interaction space. However, in a magnetron for a microwave oven, in particular, a cathode for emitting electrons is arranged on the tube axis, and a support member for supporting the cathode extends along the tube axis. Therefore, a through hole having a predetermined inner diameter must be formed at the center of a pole piece for guiding magnetic flux into the interaction space. In addition, an inexpensive and compact permanent magnet must be arranged outside a tube. Furthermore, in order to prevent electrons from flying from a portion between an end shield and corners at anode vane inner ends toward the pole pieces, magnetic flux is preferably generated obliquely with respect to the tube axis at the end portion of the interaction space. Due to the above limitations, it is difficult to obtain a uniform magnetic field distribution perfectly parallel to the tube axis over the entire range of the interaction space.
Conventionally, Japanese Patent Disclosure No. 53-38966 discloses a magnetron having a structure wherein a magnetic field is uniform or is set to be stronger at the side of the anode vanes over a range of the interaction space extending from a cathode surface to the anode vane inner end faces to improve stability of oscillation. Japanese Patent Disclosure Nos. 51-56172 and 51-58859 describes, a magnetron having pole pieces with an improved shape to obtain a parallel magnetic field distribution in the interaction space. However, in a magnetron of this type, a permanent magnet is incorporated in a tube, and pole pieces, each having substantially the same diameter as that of the magnet, are coupled to the magnet surface. Therefore, due to the structural difference, the above proposals cannot be directly applied to a magnetron having a basic structure in which a ring-shaped ferrite magnet is arranged outside the tube and magnetic flux are guided to the interaction space through funnel-shaped pole pieces.
A conventional magnetron for a normal microwave oven has a magnetic flux distribution as shown in FIG. 1 near the interaction space. Magnetic flux B are generated to be substantially parallel to tube axis Z near the center in the axial direction of interaction space S extending from substantially cylindrical electron radiation surface K to vane inner end faces A. In contrast to this, in regions Se where end seals 26 and 27 oppose the vane inner end faces, magnetic flux B are generated obliquely with respect to tube axis Z, i.e., toward center Z=0.
The conventional magnetron has a distribution shown in FIG. 2. FIG. 2 shows a relative magnetic field intensity in an interaction space between cathode surface K and anode inner end face A when an average magnetic field intensity from cathode surface K to anode inner end face A at the central portion (Z=0) of interaction space S is given by 100%, and distances along the tube axis from the central portion to respective points (Z=0, Z=.+-.1 mm, Z=.+-.2 mm, Z=.+-.3 mm, Z=.+-.4 mm, and Z=.+-.5 mm) are used as variables. As can be seen from FIG. 2, in the conventional magnetron, curves having distances Z as variables intersect each other, and intensities are substantially equal to each other regardless of distances Z in intermediate region P in the radial direction of the interaction space. Thus, the most uniform magnetic intensity distribution can be generated in intermediate region P extending along the tube axis in the interaction space. However, a large variation in magnetic field intensity occurs along the axial direction at and around anode inner end face A. The width of an anode vane, i.e., size La of an inner end face along the axial direction is normally 9.5 mm, and a magnetic field intensity difference in the axial direction around anode inner end face A reaches about 22% within this range.
As shown in FIG. 3, line noise corresponding to frequency components of 30 to 400 MHz exists at an input side through a cathode support member, and a line noise level for the low-frequency component of 30 to 150 MHz is high. For a component in a 100-MHz range (corresponding to a range of 80 to 120 MHz, and including a maximum level), the line noise level reaches about 42 dB.mu.V (decibel microvolts).
The reason for the high line noise level is that, since axial magnetic field intensities at a vane central portion and two end corners have a large difference near the anode vane inner end face in the interaction space, rotational speeds of electrons locally vary. The frequency of a high-frequency electric field induced in a resonance cavity, including the anode vanes by the electron cloud, varies depending on the position in the interaction space in accordance with the magnetic field intensity. A frequency component corresponding to a difference in frequencies is leaked to the input side as a line noise component of a relatively low frequency. This can be regarded as a cross modulation-like phenomenon. This noise level tends to be increased when a magnetron output section and a load are strongly coupled.
A strong demand has recently arisen for a compact magnetron, in particular, a small height in the axial direction in order to make a microwave oven compact. If a distance between pole pieces is simply reduced to effect magnetic flux generated from a magnet, an electric field coupling is increased between the pole pieces and strap rings. As a result, reverse emission of electrons toward a cathode is increased, and a temperature of the cathode is increased. In the worst case, thermal runaway may occur. When an axial length of the anode vane is decreased in order to keep a given distance between the pole pieces and the strap rings, load stability may be degraded. For example, it was demonstrated that if the axial length of the vane is decreased from 9.5 mm to 8 mm, the load stability of the magnetron is degraded to a peak value of 1.3 A.
Note that if a cathode introduction portion, i.e., an input stem portion is shortened, e.g., if the stem length is decreased from 20.4 mm to 10 mm, reverse emission of electrons increases a great deal, and the temperature of the cathode increases. In the worst case, the cathode may partially melt. It was demonstrated that reverse emission of electrons is increased in proportion to a decrease in stem length.